Reproduce procedure:

1. Create a new assembly
2. In the "Begin assembly" Propriety Manager, click "Create layout"
3. Draw an oblique line and make it and its two end points fully defined, by whatever method
4. Create a plane using the "Plane" tool in the sketch toolbar, Make the plane perpendicular to the line and coincident with an end point of the line.
5. Deselect anything while stay in layout mode, look at the Propriety Manager, the newly created plane is fully defined, no ( - ) before its name.
6. Double click the new plane and draw a line from the endpoint of the previous line and add a "Vertical" constraint to the new line.
7. Drag the free endpoint of the new line, you will see the new plane can rotate along with it.